Anyone who has ever stood on an asphalt parking lot in the sun understands that most dark surfaces absorb a lot of heat. In fact, a black surface can be 70 degrees F hotter than the most reflective white surfaces according to the U.S. Department of Energy. In most parts of the country, dark asphalt roofs have been the architectural norm for decades, but it may be time to rethink the aesthetic as people search for ways to lower energy usage and reduce environmental impacts.
Hot Roofs Increase Energy Consumption and Costs
It is no surprise that most roofs, which are often made of dark materials, soak up the energy of the sun and radiate it down into the structure below. That’s what makes un-insinuated attics so unbearably hot. It also heats the outside air above the roof.
Most people don’t consider, however, that their roof could actually be costing them money and increasing global warming. The heat absorbed by a dark roof is significant enough to increase air-conditioning needs and raise energy consumption. At night, the heat continues to radiate back into the environment. In densely populated areas, all those roofs combined make a big impact. According to the Heat Island Group, radiation from dark surfaces such as roofs and roadways causes the well known “heat island effect” in which cities can be 6 to 8 degrees F warmer than the surrounding areas.
Reflect Heat, Save Energy
Making a building reflective reduces the amount of solar heat it absorbs. A recent study published in Geophysical Research Letters emphasized this point announcing computer model simulations that showed if every city roof were entirely painted white, the urban heat island effect could be reduced by 33 percent.
"It remains to be seen if it's actually feasible for cities to paint their roofs white,” said the study’s lead author, Keith Oleson of the National Center for Atmospheric Research, “but the idea certainly warrants further investigation."
Observing the effect in the real world, however, both the Florida Solar Energy Center and the Heat Island Group monitored buildings with lightly colored, more reflective roofs and found that they used up to 40% less energy for cooling than buildings with darker roofs.
Cool Roof Technologies Available Now
When it comes to keeping a roof cool, reflectance is the key. And reflectance is not entirely limited by color. Materials experts have been working with the roofing industry to develop roofing materials and coatings that are more reflective than traditional products. With “cool color” technologies there are high-reflectance roofs that come in a wide variety of colors. Some even come in dark brown or near black, though they are not as reflective as the lighter options.
Cool roof shingles, stand-and-seam metal and other cool-roof surface products can be applied just like any other product when it is time to re-shingle or re-surface a roof. There are also cool roof systems that include insulation and structural components that are installed when rebuilding a roof or in new construction.
Where to Buy a Cool Roof
Many manufacturers and retailers carry cool roof products including the big hardware store chains. Some will perform better than others. So it’s important to do a little web comparison research before choosing a product.There are two widely accepted rating systems for cool roofs. The Cool Roofs Rating Council rates roofs on a scale of “0 to 1” with 1 being the coolest. The EPA Energy Star program also rates roof systems for their energy savings potential.
The cost of cool roof products is generally higher than traditional products, but according to the U.S. Department of Energy, re-roofing with cool roof technology is cheaper, faster, and provides more energy savings than planting a shade tree.
Resources:
- Cool Roofs Rating Council
- Heat Island Group a working group of Lawrence Berkeley Laboratories
Kimbra Cutlip - Kimbra Cutlip is a feature writer whose work has appeared in numerous publications including the Washington Post, Environment magazine, ...
Join the Conversation